The present invention relates to residential heating and/or cooling systems and other indoor comfort systems, and is more particularly concerned with thermostats of the type that derive the power for monitoring conditions within the comfort space and for controlling the signaling to the furnace or other comfort system from a source of thermostat power, e.g., 24 volts AC. The invention is more specifically directed to in-line controller devices which may have connection to only one end of an AC source, an example of which is a thermostat that can be used when the run of thermostat wires between the thermostat and the furnace does not include a common or C wire, and must derive its DC or battery power from the thermostat power or R wire. In that case, the associated heating apparatus may include a gas furnace, oil furnace, or electric furnace or heat pump, and cooling may be provided from a compression/condensation/expansion/evaporation cycle air conditioner, an absorption type air conditioner, a ground-water heat exchange cooling system, or other available chilling apparatus. The term “cooling” includes both sensible cooling (reducing the temperature of the comfort air) and latent cooling (removing humidity). These comfort air apparatus may have additional functions for better control of the environmental air in the comfort space, such as multiple fan speeds, high and low heating, and high and low compressor speeds, or staged capacity, and exchange of outdoor air and indoor air.
Thermostats are typically installed on an interior wall of a dwelling, business space or other residential or commercial space to control the operation of a furnace, air conditioner, heat pump, or other environmental control equipment. The thermostat continuously monitors the temperature of the room or other interior comfort space or zone, and is connected by a run of thermostat wires to the associated environmental control equipment to signal a call for heating, a call for cooling, or otherwise to keep an interior comfort space parameter, such as temperature, within some range (e.g., 68° F., ±1.0° F.). Other controls may be available, sensitive to other parameters, such as humidity or particulate level.
Older residences and other buildings had at one time used thermostats with electro-mechanical (e.g., bimetal or mercury) controls to turn the furnace and/or air conditioner on when heating or cooling was needed. Since the thermostat did not include any electrically powered control functions, the thermostat wire run between the thermostat and the HVAC equipment only needed a single thermostat power wire or R wire from one side of the thermostat transformer secondary coil, and control wires going from the thermostat to the furnace and/or air conditioner, i.e., a W wire for heat, a Y wire for air conditioning, and a G wire for fan or blower-only. The controlled elements in the furnace, e.g., gas valve relay, air conditioner compressor contactor, or fan relay, complete the circuit to the other side or return side of the thermostat transformer secondary. In more modern installations, the thermostat wire run may also include a common or C wire (usually with black or blue insulation) which connects to the return side of the thermostat transformer, so that 24 volts AC is available at the thermostat. Most modern thermostats include electronic controls that require some source of DC power. Where the thermostat wire run includes both an R wire and a C wire, then thermostat power is constantly available and can be converted to the correct level of direct current to power the thermostat. In older installations where there is no C wire, battery-powered thermostats can be employed with the DC power for the internal electronics being supplied by one, two or three power cells in the thermostat. The battery-powered thermostat is constantly drawing power from the battery supply, and after some period of time, typically about one year, the power cells become depleted and need to be replaced with fresh power cells. This is something that many homeowners and many other building occupants fail to check on and often the thermostat fails to send a call for heating or cooling to the HVAC equipment because there is insufficient battery power available in the thermostat.
In order to avoid the need for battery power for thermostats in installations where there is no C or common wire in the thermostat wire run, and there is only the single direct wire (i.e., the R wire) to the thermostat transformer, a number of thermostats have been proposed that employ a “power stealing” or “power sharing” technique to obtain DC power for the thermostat electronic controls. These thermostats direct a portion of the thermostat power (i.e., stealing, sharing, or harvesting the thermostat power) to a rectifier to use for the power for the electronic controls. That is, a small amount of current directed around the controlled switch device, i.e., triac, MOSFET, IgbT transistor, etc, to connect the R wire with the W or Y wire when there is a call for heat or cooling, but to open that connection when the call for heat or cooling is satisfied. This re-directed current, which has to be at a low level, is allowed to flow to the DC power converter and then to the W or Y wire. The current has to be small so that the current will not actuate the associated gas valve or compressor contactor. This shared power then keeps a rechargeable battery (or high-value capacitor) charged sufficiently to provide the required energy to the thermostat controls. The redirected thermostat current is available only when there is no call for heat (or cooling). In order to help power the thermostat when there is a call for heat or cooling, the power-stealing thermostat may also place a small voltage drop in line with the W or Y wire, i.e., between the thermostat and the associated load coil (gas valve relay or compressor contactor). The voltage across this voltage-drop resistor can to provide some electrical power to the DC power supply in the thermostat when there is an ongoing call for heat or cooling, while energy is being supplied to the gas valve relay or the compressor contactor. On the one hand, the amount of current flowing to the DC supply and the load coil has to be kept small so that the coil will not energize the gas valve or compressor when there is no call for heat or cooling, even at maximum transformer voltage. On the other hand, the amount of voltage drop in line with the load coil has to be kept small so that the associated load coil will not drop out when there is an ongoing call for heat or cooling, even when there is a reduced line power situation, e.g., brownout.
Additionally, many modern thermostats are so-called intelligent thermostats including programming features, environmental control capabilities, and in many cases radio, e.g., wi-fi capability, so that the thermostat settings can be changed remotely, using a cell phone or tablet, for example, or may be configured to respond to power alerts from the local electric utility. These functionalities all require the thermostat to have a reliable source of power for the thermostat electronic controls, especially for the wi-fi or other radio system, which significantly increase the DC current draw.
In most existing installations, it is difficult and time consuming to replace the thermostat wire run with a run that includes a common or C wire. In most cases it is not feasible to apply a line-current power supply or power block at the thermostat to provide DC power.
As mentioned above, in the presently available power-stealing or power-sharing techniques, the power-stealing arrangement redirects a portion of the current around the controlled switching device, i.e., around the power electrodes of the triac, directly between the R and W or between the R and Y wires. This arrangement can result in intermittent operation and, in cases where extreme cold weather causes the furnace to run continuously for long periods, can fail to keep the internal power supply adequately charged. All the prior-art arrangements are in the form of a voltage divider between the switch element and the connected load, and so the power supply is in parallel with the switch device.